1. Field of the Invention
This invention relates to an electronic test circuit that attaches to a ground fault and/or arc fault interrupt device and automatically checks for the proper functioning of the ground fault and/or arc fault interrupt device, without the need for user attention, and disables the ground fault or arc fault interrupt device in a safe condition in case of a malfunction of the device.
2. Background of the Invention
A common source of electrical injuries in the home occurs when people place radios or similar electrical devices that are operated using household AC electrical current near a swimming pool or bathtub while swimming or bathing. If a radio is knocked into water, it can create undesirable electrical leakage current through the water to ground creating what is known as a ground fault. A ground fault can also occur when a person touches an electrically hot conductor while standing on or touching a grounded conductive surface. When sufficient electrical current passes through a person, electrical burns or electrocution may result. Many electrical appliances such as heaters, hair dryers, electric razors and pumps are used near water and can present this type of hazard. Outdoor appliances such as power drills and electrically powered lawn care equipment are often subject to cut or frayed power cords that can present an electrical leakage hazard. Even a relatively low level of electrical current leakage can be dangerous to a human. Underwriters Laboratories, in their 943 standard for ground fault interrupt devices, requires that listed devices must open in response to any leakage current exceeding six milliamperes.
A ground fault is not the only class of potentially dangerous abnormal operating conditions. Another type of undesirable operating condition occurs when an electrical spark jumps between two conductors or from one conductor to ground. This spark represents an electrical discharge through the air and is objectionable because heat is produced as a byproduct of this unintentional "arcing" path. Such arcing faults are a leading cause of electrical fires. Arcing faults can occur in the same places that ground faults can occur--in fact, a ground fault would be called an arcing fault if it resulted in an electrical discharge, or spark, across an air gap. As such, a device that protects against ground faults can also prevent many classes of arcing faults.
In the United States, protection circuits known as ground fault current interrupters or GFCI's, are presently required by code for the bathrooms of most new homes and commercial buildings, as well as for garage and outdoor outlets in residential applications. In Europe, a similar device called a residual current detector, or RCD, is used for detecting and interrupting dangerous electrical leakage to ground. The theory of operation behind commercial GFCI and RCDs is the same. The major difference between the two devices is that in the U.S., most GFCIs are required to trip when leakage currents exceed 6 milliamperes while European RCDs are generally set for a trip level of 30 milliamperes.
In commercial GFCI circuits, the current carrying conductors that connect the AC source to the load will pass through a current transformer, thereby acting as primary windings for that transformer. The transformer has a secondary winding with many turns that goes to an amplifier. In a two wire system, when no electrical leakage path to ground is present, all of the electrical current that goes out one wire returns in the other wire. Accordingly, the two currents, forward and reverse, balance out one another in terms of the magnetic flux that is generated in the current transformer and so no signal is generated in the transformer secondary. On the other hand, if there is leakage to ground at the load or from the conductors connecting the source to the load, then there will be an imbalance in the currents. In other words, more electrical current goes out one wire than returns in the other, the difference being the component of current that takes another path (the ground fault current). This results in a net magnetic flux in the transformer and this will serve to generate an induced voltage in the secondary of the transformer. That secondary voltage is amplified and filtered and used to trip a relay or circuit breaker, thus removing power from the load and removing power from the leakage path. Most GFCI circuits include a reset actuator that allows the circuit breaker to be reset. Most GFCI circuits include a test button that implements an artificial leakage path around the current transformer, allowing a user to manually test the GFCI circuit, by pressing the test button and confirming operation if the internal breaker appears to open. This test does not actually confirm that an internal breaker has opened. Usually, a reset button that is mechanically connected to the relay contacts pops out and the user must assume that this means that the circuit breaker opened and the GFCI is functional.
All of the above comments also apply to GFCIs (or RCDs) that are used for multiphase systems having more than two current carrying wires. The only difference is that the additional current carrying wires are also passed through the current transformer so that in the absence of an electrical leakage path, the currents going through the transformer sum to zero.
In many electrical systems, one of the current carrying conductors will be grounded at some point in the electrical system. This grounded conductor is known as the "neutral" conductor. GFCI's will often have a circuit to detect electrical leakages from this conductor to ground by means of a second current transformer. Typically, neutral to ground faults are detected by injecting a signal onto the neutral conductor which produces an oscillation if feedback is provided through the loop completed by the neutral to ground fault.
In the United States, the National Electric Code has mandated that Arc Fault Current Interrupters, or AFCI's, be installed in certain new construction starting in the year 2002. The requirements of AFCI circuits are under development under the Underwriters Laboratories standard 1699. That standard will require that AFCI's be provided with a test circuit that simulates the arc detection circuitry output to exercise the remaining portions of the device. Since in most implementations, the AFCI will be combined with a GFCI and share much of the detection and interruption electronics, the use of a single test button for manual testing of both ground fault detection and arc fault detection will probably be common.
Ground fault current interrupts that use a differential transformer to detect the current imbalance that is indicative of a fault condition have been in use since the 1960's. U.S. Pat. No. 3,683,302 (Butler et al.) discloses a sensor for a ground fault interrupter that is operative to detect current imbalances by means of a differential transformer. U.S. Pat. No. 3,736,468 (Reeves et al.) discloses a GFCI that uses a differential sense transformer, the secondary of which is amplified to trip a circuit breaker. Other designs that combine a differential sense coil and amplifier combination to trip a circuit breaker upon fault detection include U.S. Pat. Nos. 3,852,642 (Engel et al.), 3,859,567 (Allard), 3,936,699 (Adams), 4,216,515 (Van Zeeland), 4,216,516 (Howell), 4,255,773 (Jabbal) and 4,353,103 (Whitlow).
GFCI's (or equivalently, RCD's) are required by code in many settings. In the United States, GFCI protection is required for the bathroom, garage and outdoor outlets on all new construction. The lifetime of an electrical outlet may be measured in decades but there is no assurance that a GFCI will continue to function properly over that time interval. GFCI outlets that are installed outdoors, or portable GFCI's used with construction equipment, may become encrusted with dirt or corrosion. Electronic components and insulation will age with time and this may cause a degradation in performance. A nearby lightning strike can permanently damage the fault sense electronics in a GFCI. High current demands through the outlet, even in normal use, can cause the contacts on the circuit breaker in the GFCI to weld together, subsequently preventing the circuit breaker from opening even if a fault is correctly sensed. Accordingly, there are GFCI equipped electrical outlets in service today that are not capable of delivering ground fault protection and that are thereby giving the users of such outlets a false sense of security. Although users should periodically test GFCI units, compliance is typically poor and a manual test can give misleading results or be misinterpreted by the user. As the installed base of GFCI outlets ages, the percentage of defective outlets is likely to increase. These comments apply equally to GFCI or RCD or AFCI devices used in electrical outlets, distribution panels, portable units, or units that are hard-wired into equipment.
In present day GFCI/AFCI outlets and load centers that are furnished with manual test buttons there is no way to ensure that the units are periodically tested. There is also no way to ensure that if they are tested, they are replaced if defective. Again, a manual test can give false results or can be misinterpreted by the user. Often manual testing is objectionable because if it removes power from the outlet or load center, even temporarily, it can disrupt appliances that are powered through that outlet or load center. For example, many digital clocks will be reset upon any power outage, even those caused by a temporary manual test. Accordingly, there is a need for a self-testing feature in GFCI/AFCI devices that is automatic, that is periodic, that is minimally obtrusive, and that disables the GFCI/AFCI device in a safe mode if the device cannot afford protection from a ground fault or arcing fault.
U.S. Pat. No. 4,051,544 (Vibert) describes a GFCI circuit incorporating an indicator light that turns on when, and only when, the GFCI circuit breaker opens. This invention was said to be an improvement over the prior art in that prior art devices had indicators that lit when the circuit breaker closed and thus an indicator that was burned out might lead the user to think that an outlet was dead when in fact it was electrically live. While described as a "fail safe GFCI", this invention does not check for the proper functioning of the GFCI.
The use of an automated test feature incorporated within a GFCI was suggested in U.S. Pat. No. 4,031,431 (Gross) wherein the role of the self test is to insure that the polarity on a cordset GFCI is correct for the attachment to the AC source. This invention also purportedly tests the function of the GFCI. When a control switch on the attached appliance is closed, this implements a synthetic counterbalancing fault to enable the GFCI to function and to allow power to flow to the load. A malfunctioning GFCI is indicated when the appliance continues to operate even when the control switch is in an open position. Since this testing only takes place when the appliance is manually turned off, the testing mechanism is not automatic. Using this approach does not result in fail safe operation of the GFCI.
U.S. Pat. No. 5,477,412 (Neiger et al.) describes a GFCI with built-in intelligence to detect a miswiring condition. The invention incorporates miswiring sense circuitry that automatically triggers an alarm in the case a miswiring condition is sensed. This invention does not test for the correct functioning of the GFCI.
U.S. Pat. No. 4,833,564 (Pardue et al.) describes a test circuit that adjusts the sensitivity of the GFCI to a selected threshold. The testing is not automated but is implemented when a manual test switch is engaged. The invention purportedly adjusts the magnitude of the test current so as to track changes in the sensitivity setting of the GFCI circuit. This invention does not provide for automatic testing of the GFCI, nor does it provide for fail-safe operation.
U.S. Pat. No. 5,459,630 (MacKenzie and Wafer) describes a passive test circuit that simulates neutral to ground faults and sputtering arc fault events, thus purportedly allowing a GFCI/AFCI to be fully tested. The testing is initiated through a test switch. When a test button is engaged, a bandwidth limited di/dt signal is generated and fed into the arc fault detection electronics, said bandwidth limited di/dt signal purported to simulate a sputtering arcing fault. No means is disclosed for automating these tests. No means is provided for checking for a failure in the circuit breaker.
U.S. Pat. Nos. 5,600,524 and 5,715,125 (Neiger, Gershen and Rosenbaum) describe an intelligent GFCI that periodically tests GFCI fault sensing but relies on the user to check for correct operation of the fault interruption means (the circuit breaker). This is purportedly done by setting off an audible alarm at periodic intervals, alerting the user to manually check for the proper functioning of the relay portion of the GFCI. The audible alarm then turns off when the user implements a manual test. The problems with this approach are that: (1) manual intervention is required to fully check the GFCI, with the user being responsible for recognizing whether the GFCI is correctly operating and for deciding whether to replace a malfunctioning unit; (2) with some failure modes (e.g., a welded hot side relay contact) a manual test will not detect a problem; (3) the audible alarm is obtrusive--for a residential or commercial installation with numerous GFCI's, the need to attend to beeping GFCI's might become a daily (or worse, nightly) burden; and (4) a considerable amount of consumer education will be required in order for the consumer to know that a beep means "test me" and is not an indication of malfunction. A further problem with these inventions is that the disclosed embodiments use circuit elements that are in parallel with the circuit breaker contacts, so even when the circuit breaker is open, an electrical path from source to load or source to fault will be present. In particular, if one of these parallel components were to fail in a shorted condition, opening the relay would not provide protection against a fault condition and this dangerous operating condition would not be recognized by either the self-testing electronics or by a user implementing a manual test.
A truly fail-safe fault interrupter must incorporate an emergency circuit breaker that removes power from the fault interrupter and load in the case of a malfunction by the fault interrupter. This emergency circuit breaker must be in addition to the normal or primary circuit breaker that is used to interrupt power to the load during normal operation when a ground fault condition is sensed. The reason a fail-safe fault interrupter must have this second breaker is because one common failure mode for the interrupter is when the primary circuit breaker malfunctions, thus preventing the deenergization of the load from the source. This could happen, for example, if the contacts on the primary circuit breaker were to weld together due to the so-called "hot spots" that can occur on oxidized contacts. A failure of the primary circuit breaker can also happen if the electronic switch that is used to trigger the circuit breaker fails. Malfunction can also happen due to a stuck or jammed solenoidal plunger, an open circuited or short circuited solenoid, a poor solder joint, a broken conductor or a variety of other failure sources. While the primary breaker is generally resettable, the emergency secondary breaker can be a smaller, less expensive, nonresettable "one-shot". The secondary breaker is only used to open the electrical connection when an interrupter malfunction occurs and such an occurrence will generally require that the entire unit be replaced.
A GFCI/AFCI with the self-testing feature of the present invention can eliminate the need for a manual test button or a manual reset button and may not need a resettable primary circuit breaker, but instead could use a cheaper, smaller, one-shot circuit breaker for both the primary as well as the secondary circuit breakers.
There are a variety of circuit interruption means that comprise the class of one-shot circuit breakers. The most common example is a thermal fuse, whereby two electrical conductors are in electrical contact through a low melting point linkage that opens when the current flow exceeds a certain threshold. U.S. Pat. No. 3,629,766 (Gould) describes a circuit breaker wherein a fusible wire link holds spring biased conductors in a closed position. When a predetermined electrical current is passed through the fusible link it causes it to break, effecting the snap action release of the spring arms and breaking the electrical connection. Other examples of circuit interruption means include the one-shot breaker described in U.S. Pat. No. 5,394,289 (Yao and Keung) wherein wire fuses connect two sets of two conductors. A current overload is used to break one fuse, whereupon, a cutting element is released to cut through the other fuse. U.S. Pat. No. 4,829,390 (Simon) describes a switch that is held in a normally closed position by a flash bulb. A sensor detects a dangerous condition and actuates the flash bulb, causing it to disintegrate and allowing the switch to open. Bimetallic thermal and thermal magnetic circuit breakers are well known in the art and are the basis for many resetable circuit breakers although they can be used for one-shot operation. These work by employing a blade made of two metals having different thermal coefficients of expansion. When the blade is heated, it deforms, breaking a circuit. The magnetic breakers use heating to reduce the magnetic attraction of a magnet, thereby causing a spring loaded contact to release and open a circuit. Other designs for circuit breakers include piezoelectric actuators as in U.S. Pat. No. 4,473,859 (Stone et al.) and shape memory alloy actuators as in U.S. Pat. No. 3,403,238 (Buehler and Goldstein).